diff --git a/.github/workflows/actions.yml b/.github/workflows/actions.yml
index 8ec4e7d..3173728 100644
--- a/.github/workflows/actions.yml
+++ b/.github/workflows/actions.yml
@@ -12,7 +12,7 @@ jobs:
         image: mysql
     strategy:
       matrix:
-        ruby: [ '2.7', '3.0', '3.1', '3.2' ]
+        ruby: [ '2.7', '3.0', '3.1', '3.2', '3.3' ]
         task: [ 'spec' ]
         include:
         - ruby: 2.7 # keep in sync with lowest version
diff --git a/spec/parallel_spec.rb b/spec/parallel_spec.rb
index 39e2598..2388e28 100644
--- a/spec/parallel_spec.rb
+++ b/spec/parallel_spec.rb
@@ -736,7 +736,7 @@ def cpus
     def normalize(result)
       result = result.sub(/\{(.*)\}/, "\\1").split(", ")
       result.reject! { |x| x =~ /^(Hash|Array|String)=>(1|-1|-2)$/ }
-      result.reject! { |x| x =~ /^(Mutex)=>(1)$/ } if RUBY_VERSION < "2.0"
+      result.reject! { |x| x =~ /^(Thread::Mutex)=>(1)$/ } if RUBY_VERSION >= "3.3"
       result
     end